Plasma is an ionized state of matter known for its cleaning, decontaminating, sterilizing, antimicrobial and healing properties when applied to an inanimate surface or to tissue. Plasma can be created when energy is applied to a substance. As energy input is increased the state of matter changes from solid, to liquid, to a gaseous state. If additional energy is fed into the gaseous state, the atoms or molecules in the gas will ionize and change into the energy-rich plasma state, or the fourth fundamental state of matter.
There are two types of plasma, thermal and non-thermal, which is also known as cold plasma. Thermal plasmas are in thermal equilibrium, i.e. the electrons and the heavy particles are at the same temperature. Current technologies create thermal plasma by heating gas or subjecting the gas to a strong electromagnetic field applied with a generator. As energy is applied with heat or electromagnetic field, the number of electrons can either decrease or increase, creating positively or negatively charged particles called ions. Thermal plasma can be produced by plasma torches or in high-pressure discharges. If thermal plasma is used in treating a material or surface sensitive to heat, it can cause significant thermal desiccation, burning, scarring and other damage.
In order to mitigate such damage, methods and devices have been created for applying non-thermal plasma to heat-sensitive materials and surfaces. Whereas in thermal plasmas the heavy particles and electrons are in thermal equilibrium with each other, in non-thermal plasmas the ions and neutrals are at a much lower temperature (sometimes as low as room temperature) than the electrons. Non-thermal plasma usually can operate at less than 104° F. at the point of contact. Thus non-thermal plasmas are not likely to damage human tissue.
To create non-thermal plasma, a potential gradient is applied between two electrodes. Typically the electrodes are in an environment of a fluid such as helium, nitrogen, heliox, argon, or air. When the potential gradient is large enough between the high voltage electrode and grounded electrode, the fluid between the electrodes ionizes and becomes conductive. For example, in the plasma pencil a dielectric tube contains two disk-shaped electrodes of about the same diameter as the tube, separated by a small gap. The disks are perforated. High voltage is applied between the two electrodes and a gas mixture, such as helium and oxygen, is flowed through the holes of the electrodes. When the potential gradient is large enough, a plasma is ignited in the gap between the electrodes and a plasma plume reaching lengths up to 12 cm is discharged through the aperture of the outer electrode and into the surrounding room air. The plume can be used to treat surfaces by scanning it across the surface.
Plasma systems requiring forced gas can be very large and cumbersome, requiring the use of gas tanks to supply the necessary fluid to create the plasma. Another disadvantage is that there is only a narrow contact point between the plasma plume and the surface that it comes into contact with. Typically, plumes are usually on the order of 1 cm in diameter. This makes treating larger areas time-consuming and tedious, since the contact point has to be moved back and forth across the area to be treated. The uniformity of treatment across the treatment area may be difficult to control.
Another commonly used method for creating non-thermal plasma is the dielectric barrier discharge (“DBD”), which is the electrical discharge resulting after high voltage is applied between two electrodes separated by an insulating dielectric barrier. DBD is a practical method of generating non-thermal plasma from air at ambient temperature and comes in several variants. For example, a volume dielectric barrier discharge (“VDBD”) occurs between two similar electrodes with a dielectric barrier on one electrode, and the electrodes facing each other. A VDBD is limited by the space between the two electrodes, the size of the electrodes, and cannot conform to different surface topographies. A surface dielectric barrier discharge (“SDBD”) can occur between one electrode and a surface such as skin, metal, or plastic. In a specific example of SDBD, known as a floating electrode dielectric barrier discharge (“FE-DBD”) variation, one of the electrodes is protected by a dielectric such as quartz and the second electrode is a human or animal skin or organ. In the FE-DBD setup, the second electrode is not grounded and remains at a floating potential. A SDBD treatment area is limited by the electrodes' size, and like the VDBD, it cannot conform to the surface the electrode comes into contact with. In current SDBD technologies there is only a single contact point between the plasma plume and the surface that it comes into contact with.
Another type of non-thermal plasma is known as corona discharge, which is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically charged. Corona discharges occurs at relatively high-pressures, including atmospheric pressure, in regions of sharply non-uniform electric fields. The field near one or both electrodes must be stronger than the rest of the fluid. This occurs at sharp points, edges or small diameter wires. The corona occurs when the potential gradient of the electric field around the conductor is high enough to form a conductive region in the fluid, but not high enough to cause electrical breakdown or arcing to nearby objects. The ionized gas of a corona is chemically active. In air, this generates gases such as ozone (O3) and nitric oxide (NO), and in turn nitric dioxide (NO2). Ozone is intentionally created this way in an ozone generator, but otherwise these highly corrosive substances are typically objectionable because they are highly reactive. It would be desirable to take advantage of the reactive nature of these gas molecules.
Thus there is a need for a non-thermal plasma source that can cover larger surface areas and can conform onto or around any surface topography. There is also a need for a portable non-thermal plasma source that does not require the use of pressurized gas or accompanying gas cylinders. There is a need for a non-thermal plasma source that makes good use of reactive species created by non-thermal plasma.